Chapter I. Introduction

1. Groups of galaxies

It seems to be a recognized fact that a considerable number of the
galaxies in the
general field (in this paper the term "general field" will be used to
designate the space outside the big clusters) are not isolated objects
but rather members of groups. Even
a superficial examination reveals that some of the nearby giant spiral
systems are
surrounded by clouds of smaller and larger satellites. A detailed study
of such physical groups would be of great importance, since it could
furnish valuable information
about a galaxian population in a given volume of space, especially as
regards the statistical distributions of absolute characteristics, such
as luminosities, diameters, and
masses. If the group members form a representative sample of all general
field galaxies, the results would be of universal significance. It may
not be necessary to point
out that information of this type is very hard, if not impossible, to
obtain for low-luminosity galaxies by other methods, on account of the
difficulty in determining individual distances. In the case of a
physical group the central dominant galaxy serves as a distance indicator.

In a previous investigation the writer
(1950)
has presented some results
referring to groups of galaxies. The distribution curves derived for
absolute luminosities and diameters were of a very preliminary nature,
since they referred only to members of the Local Group, the M81 group,
and the M101 group.

The present paper gives the results of a study of physical companions
belonging to 174, more or less nearby, prominent spiral galaxies. The
investigation is based on plates
previously taken with the Mount Wilson 60-inch and 100-inch telescopes
(cf.
Holmberg 1958,
1964),
and on the prints of the National Geographic Society-Palomar Observatory
Sky Atlas, the latter being evaluated down to the practical limit, as
regards galaxies. From an examination of over 3000 galaxies in circular
survey areas around the spiral systems and in nearby comparison areas, a
total of 274 physical companions has been picked out. A summary of the
material is presented in Tables 2 and
7. As will be shown in the
following sections, the analysis leads to a series of interesting
results, for instance, concerning the number of satellites, as related
to different properties of the central spiral system, and as regards the
luminosity functions of different types of galaxies.

As regards the important question whether the group members form a
representive sample of all general field galaxies, it may be stated here
that the results obtained seem to give an affirmative answer. In the present
material there are 53 spiral systems above gal. lat.
+30° out to an absolute distance
modulus of 30.0, which have altogether 82 physical companions with
absolute pg magnitudes brighter than M = - 15.0 (the observed number
out to a separation of 50 kpc has been multiplied by
3.3; cf. the next section). On the other hand, the smoothed-out space
density of galaxies brighter than M = - 15.0 is about 0.17 per
Mpc3
(cf. sect. 14 -
15), which in the
volume considered leads to a total number of approximately 180. Thus,
over 70% of all galaxies can be referred to the groups. Since the
present investigation does not claim to be complete, it seems likely
that the great majority of galaxies in the general field
are members of physical groups of the type studied here. The conclusion
is that the present material approximates to a random sample.

At this point some data will be presented for the three nearest and
best-known groups of galaxies: the Milky Way group, the M31 group, and
the M81 group. These
physical groups, which in all respects appear to be comparable to the
groups studied in the present paper, will later on be referred to for
comparison. There does not seem
to be any doubt that the Local Group is really two associations that
happen to be located rather close together: one group around each of the
two giant spiral galaxies
(there may be some uncertainty as to the status of the two Ir I systems
IC 1613 and
Wolf-Lundmark-Melotte, which are exceptionally
distant both from the Milky Way and from M31). The distances listed in
Table 1 have been taken
from the very detailed study of the Local Group by
van den Bergh (1968),
which is also the source for
the absolute magnitudes and masses of SMC and LMC; the Capricornus
system, not listed by van den Bergh, has been assumed to be at a
distance < 250 kpc on account
of the high resolution. The remaining data are from my previous works
(1958,
1964);
the log. masses in brackets have been derived from absolute luminosities
and integrated color indices by a relation given in the last-mentioned
paper. The M81 group has been studied previously
(Holmberg 1950).
A new addition is Ho IX, a
highly resolved Ir I system very close to M81 at
=
9h53m.5,
= + 69°16'(1950); the
total pg magnitude has been measured on a Mount Wilson 60-inch plate.

Table 1 probably includes all group members
outside the galactic
absorption belt that are brighter than M = - 13.5; those objects for
which information on M is not
listed (app. magnitude not accurately known) are all fainter than this
limit. It may be noted that down to an absolute magnitude of M = - 9
the three groups probably have more than 100 members (cf. the luminosity
function derived in sect. 12). In spite of
the incompleteness, the material available permits certain
conclusions. We find that
seven out of 22 members (IC 1613 and WLM not included have separations
from the central galaxy of less than 50 kpc; for the Milky Way group the
separation is assumed to be equal to the distance multiplied by
/ 4, which is the average
projection factor for a random orientation of the distance vectors The
arithmetical mean of the separations
(21 members) amounts to 111 kpc, whereas the maximum separation is about
400 kpc.